Mixing processes in general, and chemical mixing processes in particular, play an important role in substantially the entire cellulose processing industry. For example, during the bleaching of cellulose pulps, it is absolutely essential to obtain good chemical mixture in order to realize satisfactory delignification and/or bleaching thereof. The obtaining of such proper mixing of bleaching chemicals enables one to realize homogeneous bleaching results as well as good utilization of chemicals at the lowest possible reaction temperatures and shortest possible reaction times.
The "mixing problem" which to the present date has appeared to be the most difficult to solve in the cellulose industry so as to obtain acceptable results has been the mixing of chemicals to pulp suspensions at mean pulp or filter concentrations (i.e., of from about 5 to 20%). The mixing of bleaching chemicals with pulp suspensions of low concentrations, (e.g., below 5%, such as in conventional chlorination) and the addition of bleaching chemicals in a gaseous state at high concentrations (e.g., above 20%, as in gaseous phase bleaching), are, however, well-known and thoroughly tested methods which have not given rise to any appreciable mixing problems.
One problem which has been encountered during such mixing at low pulp concentrations, however, is that in view of the large liquid volumes present high pump energy is required, and in addition large amounts of emissions are effected from corresponding bleaching plants.
On the other hand, operations at high pulp concentrations (such as above 20%) means that exclusive equipment is required in order to obtain a high degree of dewatering, and the bleaching chemicals must exist as gaseous media during processing for known reasons. So-called gaseous phase bleaching at high pulp concentrations (such as above 20%) has been and still is being carried out on a technical scale for bleaching with gaseous chemicals such as oxygen, ozone, chlorine, ammonia, and chlorine dioxide.